A radioactive source has an initial activity of 6400 disintegrations per second. If the half-life is 5 years, what will be the activity after 20 years?

The number of half-lives in 20 years is 20 ÷ 5 = 4. Activity after n half-lives = Initial activity × $(\frac{1}{2})^n$ = 6400 × $(\frac{1}{2})^4$ = 6400 × $\frac{1}{16}$ = 400 disintegrations per second.